This invention relates generally to an improved arc tube configuration for a high intensity arc discharge lamp and more particularly to providing a means whereby such arc tube can be fabricated without accompanying distortion of the inner arc tube cavity.
High intensity arc discharge lamps generally employ a tubular arc tube of vitreous material as the light source. The arc tube comprises a hermetically sealed envelope, which can be quartz glass, having thermionic electrodes at opposite ends in the pinch seal regions and further contains a gaseous discharge medium which includes mercury. A typical lamp embodiment of this type is disclosed in U.S. Pat. No. 4,007,397, assigned to the assignee of the present invention, wherein the arc tube is physically supported within an outer envelope of light transmissive material by a metal mount frame and with the arc tube further including a starting electrode. The gaseous discharge medium therein disclosed can further contain alkaki metal or alkaline earth metal additives to form an amalgam with mercury. In more recently issued U.S. Pat. No. 4,581,557, also assigned to the present assignee, there is disclosed a mercury amalgam disposed within the arc tube of a high intensity arc discharge lamp so as to stabilize the electrical power being supplied to the lamp against variations in line voltage. The desired mercury amalgam is said to be formed with metals selected from the group consisting of copper, zinc, cadmium, gallium, indium, thallium, antimony and silver including combinations thereof. Both of said commonly assigned patents are specifically incorporated herein by reference since the present improvement can be applied to both type prior art lamp constructions.
Conventional fabrication of an arc tube in such type lamp constructions proceeds with a length of the light transmissive vitreous tubing having discharge electrodes centrally positioned at each end. The discharge electrodes are hermetically sealed within the central cavity formed in the vitreous tubing length by a heat sealing operation conducted at both tubing ends and wherein the molten vitreous material is pinched together mechanically with a cooperating set of jaw members. Such conventional pinch sealing operation is understandably conducted at sufficiently elevated temperatures to melt the vitreous material and form the internal cavity of the arc tube between the oppositely disposed pinch seal ends while further hermetically sealing the discharge electrodes within this cavity. A suitable gaseous discharge medium, as above indicated, is thereafter provided in the arc tube which can be supplied through separate exhaust tube means in the already known manufacturing procedures for this type lamp. A typical arc tube configuration fabricated in this manner can have the physical shape disclosed in both previously referenced patents with such arc tube construction comprising an elongated cylindrical shape aligned along a central or longitudinal linear axis. Another known arc tube configuration employs an elongated cylinder aligned along an arcuate curvilinear axis so as to provide a bowed shape member. Both type arc tube configuration desirably have the oppositely disposed principal discharge electrodes aligned along the longitudinal axis of the arc tube.
Several problems still exist with conventional lamp manufacture in the foregoing manner which can deleteriously effect optimum and reliable lamp performance. All of these problems relate to an objectionable flow of vitreous material when the arc tube cavity or chamber is formed by pinch or press sealing means. More particularly, both the shape and volume of the arc tube cavity can be significantly distorted or altered with an overflow of the vitreous material and to a degree which disturbs the lamp performance. For example, a variation in the arc tube volume alters the dosage for the gaseous discharge medium which in turn can influence the operating voltage for this type lamp. A distortion in the ends of the arc tube chamber produces other undesirable effects upon the lamp performance. First of all, significant distortion at the chamber ends can reduce control of the lamp operating voltage as well as reduce control of the lamp operating temperature. Since lamp temperature dictates efficacy and color provided with the illumination, it can be appreciated that end chamber distortion reduces control of important factors in the lamp operation. Proper temperature control at the arc chamber ends is important in regulating condensation of the solid components in the gaseous discharge medium during the lamp operation. It follows from such considerations that both arc tube chamber size and shape must be carefully controlled in providing reliable and optimum lamp performance. Still other important considerations apply for proper construction of the arc tube member itself. For example, any significant distortion of the arc tube cavity during the pinch sealing can compromise a desired positioning of the discharge electrodes. The mechanical strength of the arc tube would also be influenced by distortion of the internal cavity.
Accordingly, one object of the present invention is to provide improved means whereby greater control is exercised over the physical configuration of a particular arc tube member being employed in a high intensity arc discharge lamp.
Another object of the invention is to provide an improved arc tube construction which includes physical means better avoiding distortion during the arc tube fabrication.
A still further object of the invention is to provide a high intensity arc discharge lamp employing specific arc tube means.
Still another object of the present invention is to provide a relatively simple modification in otherwise conventional lamp manufacture enabling improved construction of a particular lamp arc tube member.
These and still further objects of the present invention will become apparent upon considering the following detailed description for the present invention.